1. Field of Invention
This invention relates to a coaxial cable useful as the feeder cable of an automobile antenna.
2. Description of Related Art
FIG. 1 is a perspective view showing a conventional coaxial cable having a shielding braided wire tube as an external conductor to reduce inductive interference and the resulting external noise on the receiving radio waves. A central conductor 10 held in a cylindrical insulator 12 extends along the length of the cable. The conductor 10 is made of steel wire or another substance having superior electrical conductivity. The cylindrical insulator 12 is made of resin such as polyethylene. A braided wire tube 14 for blocking external noise extends along the outer circumference of the insulator 12 as the external conductor. A cylindrical insulating cover 16 completes the cable.
As shown in FIG. 2, in the typical coaxial cable, the braided wire tube 14 is formed using four thin conducting wires a, b, c, and d having a diameter of 0.14 mm each and arrayed in parallel to each other to form a set of element wires (a strand). By braiding, for example, sixteen sets of such strands 20, 21, 22, 23, 24, 25, . . . so as to cover the outer circumference of the insulator 12. The braided wire tube 14 is coaxial with the central conductor 10 and the insulator 12 is interposed between the tube 14 and conductor 10. A part of the tube 14 is grounded to shield against inductive interference. An example of a cable made using a plurality of fine braided wires as a part of a shield is shown in U.S. Pat. No. 2,028,793. U.S. Pat. No. 4,552,989 also appears to describe a shielded coaxial cable made using a plurality of fine wires.
Conventional coaxial cables with the described structure have certain problems. For instance, the conducting wires a, b, c, and d have a diameter of about 0.14 mm each. Because the wire diameter is very small, the conducting wires are frequently broken or cut during the braiding process, thereby reducing product yield. Also, the four conducting wires a through d of the braided wire tube 14 are wound on bobbins and then braided. The wire break detection sensor cannot detect broken wires unless all four conducting wires are cut and so broken wire defects are not accurately detected. As a result, defective products having one or two broken wires in the braided wire tube 14 are mixed with non-defective products, thus causing a high rate of non-uniformity in product quality.
Another problem occurs during the processing of the end of the coaxial cable. FIG. 3 shows an example of coaxial cable end processing. The portion 14A to be processed at the end of the braided wire tube 14 is shown peeled and turned back toward the outer cover 23. The portion 14A is to be soldered to a grounded conductor. When the portion 14A is peeled back, one or more of the thin conducting wires a, b, c, and d forming the braided wire tube 14 often fall into the space between the braided wire tube 14 and the insulator 12, as shown by a filament 14B in FIG. 3. The thin filament 14B is often overlooked and frequently causes a short-circuit with the central conductor 10 during use. To prevent such a short-circuit, double or triple inspections must be made for the presence of such filaments 14B, thereby reducing productivity.
According to French Pat. No. 873,673 a flat conductor is wound around the cable to form the tube. According to U.S. Pat. No. 3,240,867 a tube is shielded with a braided metal cover for inclusion in an extensible cable. Braiding flat material around a wire is also shown in U.S. Pat. No. 2,863,032 for use in an insulated heater wire.